Guard average molar mass and validate materials depletion inputs

[approx-infinity]

Description

• Raise a clear error if average_molar_mass is accessed before any nuclides are added.
• Add validation for Materials.deplete input lengths and missing material volumes.
• Fix a small docstring typo in Materials.deplete.
• Add unit tests covering the new validation errors.

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 18) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[paulromano]

Thanks @approx-infinity!